How to counter Turks fast-castling to mass janissaries?

I just lost two games in Arabia in a row where the opponent plays Turk and builds a mass army of janissaries after reaching castle age quickly. I didn't realize that they outrange most units, so I was a bit shocked to find out how strong they are in castle age, even though you often hear negative things about Turks as a civ in general (e.g. gold-heavy, strong in imp, etc.). If the opponent reaches the castle age in similar time as you do, and he is already fully walled up, what can you do? Especially after an early feudal rush fails to put a break on the opponent's economy.